I do not own a n9 but even my spare n8/n900 serve on regular basis as:

- Timelapse device if you have some work to document
- Babyphone
- Surveillance- or Webcam
- mobile wifi data storage
- spare data sim hotspot
- Dedicated Navi to not get interrupted by calls
- torch and car youtube device for the kids
- webradio (mostly SomaFM)

So, after a towel, i can not imagine a more universal helper than a spare smartphone!